In a communication system such as a direct sequence spreadspectrum code division multiple access (DS-CDMA) system, a received signal at a base station comprises a multiplicity of frequency and time overlapping coded signals from individual remote units each of which has undergone multipath scattering. Each of these signals is transmitted simultaneously at the same radio frequency (RF) and is distinguishable only by its specific encoding. In other words, the uplink signal received at a base-station receiver is a composite signal of each transmitted signal and an individual remote unit's signal is distinguishable only after decoding.
In conventional DS-CDMA systems, the receiver decodes each remote unit separately by applying each respective remote unit's code to the composite received signal. Each individual remote unit's signal is thereby "despread" from the composite received signal. Due to the nature of the family of codes utilized, the other remote units' signals remain in a spread form and act only to degrade the recovered signal as uncorrelated interference.
Prior art techniques of interference cancellation are known to reduce even uncorrelated interference. This permits an increase in the sensitivity and or capacity of the multi-remote unit system. The most common technique of interference cancellation is to synthesize a replica of a particular remote unit's received signal, after it has been properly decoded, and utilize the synthesized replica to cancel interference (by subtraction) from the received signal. Such a prior-art method of interference cancellation is described in US Patent "Method and Apparatus for Canceling Spread-Spectrum Noise" by Stilwell et. al., (U.S. Pat. No. 5,235,612) assigned to the assignee of the present invention, and incorporated herein by reference. Because serving base stations utilizing prior-art techniques of interference cancellation synthesize a replica of a particular signal only after it has been properly decoded, the serving base stations can only eliminate interference from signals of remote units actually in communication with the serving base stations (i.e., within the serving base station's coverage area). Therefore, by utilizing such prior-art techniques, it is possible for serving base stations to effectively eliminate a remote unit's signal from the composite received signal only if the remote unit happens to be in communication with the serving base station.
In a cellular environment, remote units in communication with nonserving base stations and not communicating with the serving base station contribute as much as half the noise energy to a serving base station. Therefore, prior-art techniques of interference cancellation are deficient in that they fail to eliminate any interference of remote unit's communicating with non-serving base stations and not in communication with the serving base station. Therefore, a need exists for a method and apparatus for canceling interference of signals generated in non-serving cell sites.